As is known in the art, electromagnetic interference (EMI) filters are used in many types of electrical equipment. Such filters play a role in meeting federal regulations for device compatibility, for example. The size and performance of these filters are often limited by filter capacitor parasitics, such as the equivalent series inductance (ESL) associated with magnetic energy storage in the capacitor and interconnects. Coupled magnetic windings can be used to compensate for the effects of parasitic inductance of capacitors, reducing the filter volume and increasing its attenuation performance.
FIG. 1A shows a prior art LC low pass filter LPF having a capacitor C and an inductor L. FIG. 1B shows the filter of FIG. 1a with a high-frequency model 20 of the capacitor C. The high frequency capacitor model 20 includes the equivalent series inductance ESL and resistance R of the capacitor. These parasitics cause non-ideal filter behavior at high-frequencies as the capacitor impedance is no longer dominated by its capacitance. The filter attenuation performance is determined by the impedance mismatch between the series and shunt paths. At frequencies beyond the resonant frequency of the capacitor C, the shunt path impedance increases, and the filter LPF performance degrades as compared to the ideal LC filter.
It is known to use coupled magnetic windings to compensate for the effect of the parasitic inductance of the filter capacitor, greatly improving high-frequency filter performance. U.S. Pat. No. 6,937,115 to Perreault et al., for example, which is incorporated herein by reference, discloses a coupled inductor structure used to induce a voltage that counteracts the voltage due to the capacitor equivalent series inductance. This effect arises from the mutual coupling of the magnetic windings and appears as a negative branch inductance in the equivalent “T” model of the coupled magnetic windings. The negative branch inductance is used to compensate for the device parasitic inductance, creating a three terminal component with drastically reduced equivalent shunt path inductance.
FIGS. 2A and 2B show a prior art compensated LC low-pass filter having inductance cancellation with the coupled magnetic winding represented by its equivalent “T” model. An implementation of this inductance cancellation method has been found to be effective for improving the performance of high frequency filters, such as those used in limiting electromagnetic interference (EMI). FIG. 2A shows an unsimplified equivalent circuit for the filter and FIG. 2B shows an equivalent simplified version.
While the inductive component of a single capacitor can be cancelled as described above, providing coupled windings for each capacitor in a circuit can require significant real estate.